All actually last about 6 months, split into two 3-month sections. Most allow you to delay your exam sittings while you consolidate with the work (we certainly do). Some will give you a little support during this time, most offer extra tuition of some sort if you pay. The workload is high, but not impossible for most people.
When you talk to each school (there are at least 6 full-time modular schools I can think of approved through the CAA (assuming BAe has restarted modular), and I would suggest you contact them all) ask about their policy on delaying exams, and support offered. Some may restrict due to numbers of students on the following courses you would have to join after delaying your first set of exams.